A variety of methods are available for fabricating complex microscopic and mesoscopic parts out of ceramics and metals, in which feature or part sizes are below 1 millimeter. These sizes are not accessible by conventional machining techniques, including milling and turning. Micromachining techniques can be divided into a few main groups: thin film techniques, micro-stereolithography (.mu.-SLA), photo-chemical etching, and LIGA.
Thin film techniques include etching, sputtering, and chemical vapor deposition. Material is deposited onto a silicon substrate, which is etched away after the part is formed. Complex multi-material assemblies can be produced with high accuracy. However, only certain materials can be deposited in sufficient quality with these techniques. Feature height is limited to about 10 .mu.m, above which internal stresses built up during thin-film deposition cause delamination. In addition, microscopic parts formed by this technique cannot be combined with conventionally machined parts to form an assembly. A related technique involves electroplating into patterned silicon substrates. Generally, a thin seed layer is deposited onto the wafer as an electroplating base before electroplating. This technique also has height limitations, and complex structures with overhangs cannot be electroplated.
SLA is widely used for manufacturing macroscopic prototypes out of polymeric materials, and has been adapted for micropart fabrication, as described in T. Nakamoto et al., "Manufacturing of three-dimensional micro-parts by UV laser induced polymerisation," Journal of Micromechanics and Microengineering, Vol. 7, pp. 89-92 (1997). Parts are made by scanning a UV laser over a photo-curable resin. In regions where the laser penetrates the liquid surface, the resin solidifies and forms the part. SLA achieves complex three-dimensional structures, but accuracy is limited by the spot size of the laser and its penetration depth. It is also difficult to remove the uncured resin from microscopic cavities.
In photo-chemical etching, a focused laser beam removes material by photo-chemical ablation. This process has been used to shape silicon with an argon-ion laser in a chlorine atmosphere, and is described in T. M. Bloomstein and D. J. Ehrlich, "Laser deposition and etching of three-dimensional microstructures," IEEE, Transducers '91, pp. 507-511. Laser ablation allows the creation of real three-dimensional structures. Its main disadvantage is the slow speed of the process. Since surfaces must be scanned in a serial fashion, photo-chemical etching cannot compete with the highly parallel lithographic processes, especially when a large number of devices are manufactured or if large volumes of material must be removed.
LIGA (Lithographie, Galvanik, Abformung) involves the use of a bright X-ray beam to irradiate polymers such as polymethyl methacrylate (PMMA). The irradiated PMMA degenerates into shorter polymer chains that are soluble in certain solvents. By covering some regions Qf the polymer with an X-ray mask, microstructured polymer parts can be manufactured. LIGA can produce polymer microstructures with very high precision and high aspect ratios. By electroplating into the polymer mictrostructures, metal parts can be formed. However, standard LIGA cannot be used to produce complex, multi-layered parts.
A method for producing complex structures using PMMA sheets patterned by LIGA has been disclosed in U.S. Pat. Nos. 5,378,583 and 5,496,668, issued to Guckel et al. Multiple layers can be stacked to form complex microstructures of up to 1 mm in height. This method has some significant drawbacks, however. Both X-ray masks and access to the synchrotron light source needed for X-ray generation are extremely expensive. In fact, while the LIGA method is, in theory, capable of highly parallel manufacturing of numerous identical or different parts, incorporating an X-ray source into mass production is unfeasible. Furthermore, part material that can be cast or plated into polymer microstructures is quite limited. The mold must be able to withstand the filling process without heat- or pressure-induced deformation, and the part material must be unaffected by the method (usually chemical) used to remove the polymer. The thin, flexible polymer is also difficult to handle and align, with both the X-ray mask and subsequent layers. Polymer layers also cannot be combined with macroscopic layers to form macroscopic parts incorporating microscopic features.
There is still a need for a process for accurately fabricating complex, three-dimensional microstructures that can be used for mass production.